1. Field of the Invention
The present invention relates to an optical information recording method for recording information to an optical information recording medium through the use of holography, an optical information reproduction method for reproducing information from an optical information recording medium through the use of holography, and an optical information recording/reproduction method for recording information to an optical information recording medium and reproducing information from an optical information recording medium through the use of holography.
2. Description of the Related Art
Optical information recording/reproduction wherein information is recorded to and reproduced from an optical information recording medium through the use of holography has been implemented conventionally. In general, holographic recording for recording information to a recording medium through the use of holography is performed by superimposing information light which carries image information onto reference light within the recording medium and writing an interference pattern generated as a result to the recording medium. When reproducing the recorded information, a reproduction light which carries image information is reproduced by diffraction due to interference pattern, by irradiating the recording medium with reference light.
In recent years, volume holography, digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. Volume holography is a system wherein an interference pattern is written three-dimensionally by making positive use of the recording medium in a direction of its thickness as well, and is characterized in that diffraction efficiency can be enhanced by increasing thickness and recording capacity can be increased by utilizing multiplex recording. Digital volume holography is a computer-oriented holographic recording system which, while using the same recording medium and recording system as in the volume holography, limits the image information to be recorded to a binary digital pattern. In digital volume holography, even analog image information such as a picture, for example, is once digitalized and developed into two-dimensional pattern information (also referred to as two-dimensional digital page data), which is then carried by information light, allowed to interfere with reference light, and recorded as a hologram. For reproduction, this digital pattern information is read and decoded to restore the original image information for display. Therefore, even if the SN ratio (signal-to-noise ratio) during reproduction is poor, the original information can be reproduced with extremely high-fidelity by performing differential detection or error correction on binary data which has been encoded.
In conventional optical recording/reproducing devices, two-dimensional digital pattern information is displayed in a spatial light modulator (may be abbreviated as SLM) which has a large number of pixels arranged in a grid, and information light carrying the two-dimensional digital pattern information is generated by changing the state of the light, such as the phase, intensity, and wavelength, per pixel with the spatial light modulator. What is actually written to the recording medium is not the two-dimensional digital pattern information per se, but the interference pattern of information light and reference light.
In addition, the encoding method for two-dimensional digital pattern information conventionally expresses one-bit digital information, “0” or “1”, by two pixels in the spatial light modulator, and invariably, one pixel out of the two pixels corresponding to the one-bit information is “ON” and the other, “OFF”. If both of the two pixels are either ON or OFF, it becomes an error data. Thus, expressing one-bit digital information by two pixels can increase detection accuracy of the data through differential detection.
Generally, in the information storage field, recording density and transfer rate are important factors. In digital volume holography, high recording density and high transfer rate have been realized by the following methods.
First, the recording capacity per volume, namely recording density, can be enhanced by performing multiplex recording of two-dimensional digital pattern information in a volume recording medium. Next, the number of bits recorded or reproduced per second which is determined by (the number of vertical pixels in the two-dimensional digital pattern information)×(the number of horizontal pixels)×(the number of frames), namely the transfer rate, can be increased by improving the frame rate (the speed by which pixels are displayed or display is detected) of the spatial light modulator which displays the two-dimensional digital pattern information and the detector which detects the reproduced two-dimensional digital pattern information, when recording information.
For example, Japanese Patent Laid-Open Publication No. 11-311937 describes techniques such as phase encoding multiplexing wherein a plurality of information are superimposed and recorded by changing phasing signals, hole burning-type wavelength multiplexing wherein a plurality of information are superimposed and recorded by changing wavelengths, shift multiplexing wherein a plurality of interference areas are arranged so as to be out of alignment to each other in a horizontal direction and partially overlapping, and a plurality of information are superimposed and recorded, as multiplex recording in an optical recording reproduction device which utilizes digital volume holography.